Generally, a mechanical grit chamber, which is installed at an entrance of a conventional small or medium-sized wastewater treatment plant, comprises a rectangular wastewater tank made of steel. While wastewater introduced into the wastewater tank flows to be discharged, due to a flow velocity of wastewater flowing in the wastewater tank, sand of no less than a predetermined grain size sediments, and organic material and sludge of no greater than the predetermined grain size are discharged out of the wastewater tank without sedimenting. However, in this mechanical grit chamber, specifically other than a civil-engineered grit chamber, a high partition wall must be installed adjacent to an exit of the wastewater tank to increase a retention time of wastewater over a short distance so that wastewater can overflow the partition wall to be discharged out of the wastewater tank. The grit chamber is designed such that wastewater can pass through the wastewater tank at a flow velocity of about 0.3 m/sec when assuming a maximum inflow amount, and that sand having a grain size of no less than about 0.2 mmΦ can sediment for a retention time of about 30˜60 seconds. If an inflow amount of wastewater decreases, a passage velocity of wastewater naturally decreases in proportion to the inflow amount of wastewater, by which a problem is provoked.
A decrease in a passage velocity of wastewater means that wastewater stays in the wastewater tank for a time which is longer than a designed appropriate retention time, as a result of which even the sand having a grain size of no greater than about 0.2 mmΦ and organic material having a low specific gravity can sediment in the wastewater tank.
Concretely speaking, it is the norm that an amount of wastewater sharply varies by various factors. Specifically, the discharge of sewage which occupies the most of the wastewater abruptly increases after a breakfast meal and an evening meal among day times. Therefore, when designing a grit chamber, a flow velocity of wastewater cannot but be determined by assuming a maximum inflow amount of wastewater. During the other times which correspond to about 90% of a day, an inflow amount of wastewater reaches only about 10˜20% of the maximum inflow amount of wastewater. Therefore, during most times of a day, a flow velocity of wastewater which passes through the wastewater tank reaches only about 1/10˜⅕ of an originally designed flow velocity.
Hence, as organic material and sludge, which must not sediment but pass through the wastewater tank for a next process, sediment in the grit chamber together with sand, is pushed toward an inlet by a horizontal conveyance screw, and then removed out of the wastewater tank by a take-out screw which is installed at an inclination angle of 30. Since the sedimented mixture has flowability such as slurry, in the case of a shaftless take-out screw which is mainly adopted, the sedimented mixture streams downwards profusely through a shaftless portion defined in the center of the screw while being pushed upward, whereby the mixture cannot be properly removed. Also, even in the case that a take-out screw having a shaft portion is applied, in order to take out sand having a certain grade of dryness, the take-out screw must be designed to rotate at a low speed with a high torque, by which most of the sedimented mixture is likely to stream down between the take-out screw and an outer casing before arriving at the top of the take-out screw, whereby the mixture cannot be properly removed. Hence, if the take-out screw having the shaft is designed as a screw pump to rotate at a high speed with a low torque, quick abrasion of a lubrication lining intervened between the take-out screw and the casing is caused. Also, if sand having a high grade of dryness is taken out as an increased amount of sand is introduced into the wastewater tank, since the torque of a driving motor is small, the take-out screw is apt to be stopped due to a drag force induced by the casing and the sand, whereby breakage of a mechanical operating system may result. In this regard, it is not economic to adopt a motor which can generate significantly high power to provide a high torque.
In order to cope with these problems, a bucket type grit chamber can be applied in place of the screw type grit chamber to efficiently remove the mixture. Nevertheless, even in this case, since the mixture containing organic material and sludge has flowability such as slurry, the amount of the mixture increases up to three times when compared to the case in which only sand is taken out. Also, it is difficult to deliver the mixture using a conveyor since the mixture streams like water, and when storing the mixture, the mixture is apt to leak out of a storage hopper. The transportation of the mixture costs two or three times an expense which is incurred when transporting only the sand. Further, the mixture can be rejected as being improper to be reclaimed at a reclaiming site, whereby reliable operation of the grit chamber cannot be ensured. That is to say, as the grit chamber is left alone without being operated, wastewater processing efficiency cannot but be deteriorated at downstream wastewater treatment plants. Since these problems also occur in the civil-engineered grit chamber, it is to be understood that most grit chambers of wastewater treatment plants cannot but be abnormally operated.